Yesterday, quite coincidentally, two of my colleagues sent me electronic communications and used the word bumf. (This is not a reflection of their opinion of my work, of course.) But what was interesting was that both of them mis-spelt it bumpf.
Bumf is a 'clipping' or shortening of the word bumfodder, and both used to mean 'toilet paper'. I always thought it was Dutch and have just this very moment discovered that it is not. It's dated back to 1889 and is given as 'British schoolboy slang'. This mis-spelling as bumpf might come from mixing up the two spellings: it can also be bumph.
Bum-fodder is three syllables, with the first two divided between the /m/ and the /f/. This is unremarkable: if you have an /m/ and an /f/ together in English, they're normally either side of a syllable boundary. Ham-fisted, chamfer and bumface are all other examples of this. But when it's shortened, it's just one syllable, so that single syllable ends in the consonant cluster /mf/. This is a bit unusual in English. Try and think of other words that end in this combination. There's a few, but they're rare. The OED has nymph, galumph, wumph, triumph and harrumph, among others. And they're all spelt with a 'ph'. I don't have an explanation of this spelling quirk (some come from Greek, which is where most of our 'ph' spellings come from, but not all). I can tell you why that rogue 'p' gets into bumpf though.
Basically, /m/ and /f/ are very nearly as different as two sounds can be. This means that when we say them next to each other, we add in another sound that's halfway between them to make the transition a bit easier. /p/ is made with the same lip-shape as /m/, and the airflow is the same, but the vocal folds are like they are when we say /f/. This insertion of a sound is called 'epenthesis' and we do it all the time: adding a 'p' in 'hamster' is a famous one, showing that this isn't because the cluster comes at the end of the syllable. It only occasionally shows up in spelling mistakes (like bumpf or hampster), and this is a really cool insight into what we say and what we are aware of when we say it.